The present invention relates to a method for conversion of linearly encoded digital signals into non-linearly encoded digital signals according to a multiple segment characteristic obeying the A law or the .mu. law. This A law and .mu. law are well known in the art of handling pulse code modulated (PCM) signals in telephone systems.
When samples of analog signals obtained by periodic sampling are converted into digital signals in the form of PCM words, the analog values which fall within a continuous amplitude range must be correlated to a limited number of amplitude levels because of the limited number of coding digits used. Hence, a so-called "quantization" takes place. As a consequence of such quantization, quantization noise results. To ensure that this quantization noise will not be noticable as a disturbance, it is necessary to maintain a certain ratio of analog signal amplitude to the quantization-caused interference amplitude.
When using 4000 amplitude levels uniformly distributed over the total amplitude range of the analog signals, it would indeed be possible to obtain an adequate signal-to-noise ratio. It would then be necessary, however, to transmit digital signals having at least twelve code character elements or digits. Furthermore, with such a uniform level distribution the S/N ratio would be unnecessarily high in the range of high analog value amplitudes. In the course of encoding, therefore, a so-called "companding" is carried out; that is, the linear code representation is converted to a non-linear code representation in such a way that the signal-to-noise ratio is constant over the entire amplitude range. An exactly constant signal-to-noise ratio results in a logarithmic companding characteristic. In practice one operates according to companding characteristics that are easy to realize and which obey the so-called A law or the so-called .mu. law. When using the A law, one operates according to a characteristic which is composed of thirteen rectilinear segments; with the .mu. law, there are fifteen segments. The slope of the line of these characteristics decreases in each of the two half-planes from segment to segment by a factor of 2. Each segment, in turn, is divided into sixteen quantization steps of equal size, whose height increases from segment to segment by a factor of 2. This regularity is interrupted in the first segment. When taking the A law as a basis, the first segment contains 32 positive and 32 negative quantization steps, each encompassing two values of the linearly encoded information. In the .mu. law the first segment is divided into fifteen positive and fifteen negative quantization steps, likewise encompassing two values of linearly encoded signals, as well as into a positive and a negative quantization step encompassing only one value of the linearly encoded signal. Thus in the A law 8192 steps in linear encode representation are correlated with 256 steps in companded representation. In the .mu. law the ratio of linearly encoded steps to non-linearly encoded steps is 16318 to 256.
If the mentioned correlation of linearly encoded signal values to non-linearly encoded signal values is effected with the aid of a memory, on the basis of the A law 2.sup.13, and on the basis of the .mu. law 2.sup.14 addresses are needed to activate the 256 possible non-linearly encoded signal values. Because of the eight bits required for non-linear code representation, this means a memory capacity in the first case of 2.sup.16, and in the second case of 2.sup.17 bits or, in other words, 64K bits and 128K bits, respectively. When using quantity-produced, integrated semiconductor memories of suitable size, in particular those with 512 storage places of eight bits each and this a storage capacity of 4K bits, it is necessary to provide sixteen or, respectively, thirty-two such memories, each having twenty connections. However, this large number of required connections for all the memories is undesirable in practice and expensive.